When using systemsettings/Display and Monitor/Display/Scale display the scale factor is set and work. OK. But it is not possible to reverts this setting! When choose these menu again after you scale your display to eg. 1.2 the slider for the scale factor is set to 1.0 and you are not possible to revert the scale factor below 1. :-\ KDE Plasma Version: 5.8.7 KDE Frameworks Version: 5.32.0 Qt Version: 5.6.2 ADDITIONAL INFORMATION A hint in which config file these setting is saved will be neccessary if these versions are not fixed.
Fixed in a later version of Plasma. 5.8.7 is quite old at this point, and you might consider upgrading to the latest LTS (5.12) or even the newest release (5.14).